1. Field of the Invention
The invention relates to a silicon crystallization method and a silicon crystallization apparatus, in which it is possible to form an alignment key without additional photolithography, and to adjust the substrate to a correct the position by sensing a deviation of the substrate when the substrate is loaded.
2. Discussion of the Related Art
Various displays increase in demand as information technologies develop. Recently, many efforts have been made to research and develop various flat display panels such as a liquid crystal display device (LCD), a plasma display panel (PDP), an electroluminescent display (ELD), a vacuum fluorescent display (VFD), and the like. Some types of the flat display panels have already been used in various display devices.
LCDs are most widely used because of their beneficial characteristics and advantages including high quality images, light weight, thin and compact size, and low power consumption. LCDs can be used as substitutes for cathode ray tubes (CRT) for mobile image display devices. LCDs have also been developed for use in devices receiving and displaying broadcast signals, such as televisions, computer monitors, and the like.
Generally, an LCD device includes an LCD panel for displaying an image and a driving unit for applying driving signals to the LCD panel. The LCD panel includes first and second glass substrates bonded to each other and a liquid crystal layer injected between the first and second substrates.
In this case, on the first glass substrate (TFT array substrate), gate lines are formed to be arranged in one direction at fixed intervals, data lines are arranged perpendicularly to the gate lines at fixed intervals, pixel electrodes are formed in a matrix-type configuration in pixel regions defined by the gate and data lines crossing each other, and thin film transistors are switched by signals of the gate lines to transfer signals from the data lines to the pixel electrodes.
On the second glass substrate (color filter substrate), there is a black matrix layer for shielding light from other portions except the pixel regions. On the second glass substrate can also be found an R/G/B (Red/Green/Blue) color filter layer for realizing colors, and a common electrode for realizing an image.
The above-described first and second glass substrates are maintained at a predetermined interval from each other by spacers, and the substrates are bonded to each other by a sealant having a liquid crystal injection inlet. Liquid crystal material is injected between the two glass substrates.
The general driving principle of an LCD device uses the optical anisotropy and polarization characteristics of the liquid crystal. Because the structure of a liquid crystal molecule is thin and long, the liquid crystal molecules are aligned along a specific direction. Based upon dipole moment, the liquid crystal molecules can have either positive or negative dielectric anisotropy. Applying an induced electric field to the liquid crystal controls the direction of the alignment. Therefore, when the alignment of the liquid crystal molecules is arbitrarily controlled, the alignment of the liquid crystal molecules eventually alters. Subsequently, due to the optical anisotropy of liquid crystals, light rays are refracted in the direction of the alignment of the liquid crystal molecules, thereby representing image information.
In recent technologies, an active matrix liquid crystal display (LCD), which is formed of a thin film transistor and pixel electrodes aligned in a matrix form and connected to the thin film transistor, is considered to have excellent high resolution and is noted for its ability to represent animated images.
In an LCD device having a polysilicon semiconductor layer of the thin film transistor, it is possible to form the thin film transistor and a driving circuit on the same substrate. Also, there is no requirement to connect the thin film transistor with the driving circuit, whereby the fabrication process is simplified. In addition, the field effect mobility of polysilicon is one to two hundred times higher than the field effect mobility of amorphous silicon, thereby obtaining a great stability to temperature and light.
The method of fabricating the polysilicon can be divided into a low temperature fabrication process and a high temperature fabrication process depending upon the fabrication temperature.
The high temperature fabrication process requires a temperature of approximately 1,000° C., which is equal to or higher than the temperature required for modifying substrates. Glass substrates have poor heat-resistance, and hence expensive quartz substrates having excellent heat-resistance should be used. When fabricating a polysilicon thin film by using the high temperature fabrication process, inadequate crystallization may occur due to high surface roughness and fine crystal grains, thereby resulting in poor device characteristics, as compared to polysilicon formed by the low temperature fabrication process. Therefore, technologies for crystallizing amorphous silicon, which can be vapor-deposited at a low temperature to form polysilicon, have been researched and developed.
The method of depositing amorphous silicon at low temperature, and crystallizing the deposited amorphous silicon, can be categorized as a laser annealing process and a metal induced crystallization process.
The low temperature laser annealing process includes irradiating a pulsed laser beam on a substrate. More specifically, by using the pulsed laser beam, the solidification and condensation of the substrate can be repeated about every 10 to 100 nanoseconds. The low temperature fabrication process has the advantage that the damage caused on a lower insulating substrate can be minimized.
The related art crystallization method of silicon using the laser annealing method will now be explained in detail.
FIG. 1 illustrates a graph showing the size of amorphous silicon particles versus laser energy density.
FIG. 1 shows that the crystallization of amorphous silicon can be divided into a first region, a second region, and a third region depending upon the intensity of laser energy.
The first region is a partial melting region where the intensity of the laser energy irradiated on the amorphous silicon layer is sufficient to penetrate only the surface of the amorphous silicon layer. After irradiation, the surface of the amorphous silicon layer partially melts in the first region, and small crystal grains are formed on the surface of the amorphous silicon layer after the solidification process.
The second region is a near-to-complete melting region where the intensity of the laser energy, being higher than that of the first region, almost completely melts the amorphous silicon. After melting, the remaining nuclei are used as seeds for crystal growth to thereby form crystal particles with an increased crystal growth, as compared to the first region. However, the crystal particles formed in the second region are not uniform. The second region has a narrower laser energy density band than the first region.
The third region is a complete melting region where laser energy with an increased intensity, as compared to that of the second region, is irradiated to completely melt the amorphous silicon layer. After the complete melting of the amorphous silicon layer, a solidification process is carried out to allow homogenous nucleation, thereby forming a crystal silicon layer formed of fine and uniform crystal particles.
In this method of fabricating polysilicon, the number of laser beam irradiations, i.e., shots, and the degree of overlap are controlled in order to form uniform, large and rough crystal particles by using the energy density of the second region.
However, the interfaces between the many polysilicon crystal particles act as impediments to electric current flow, thereby decreasing the reliability of the thin film transistor device. In addition, collisions between electrons may occur within the many crystal particles to cause damage to the insulating layer due to the collision current and deterioration, thereby resulting in product degradation or defects. In order to resolve such problems, fabricating polysilicon uses a sequential lateral solidification (SLS) method, where the crystal growth of the silicon crystal particle occurs at an interface between liquid silicon and solid silicon in a direction perpendicular to the interface. The related art SLS crystallizing method is disclosed in detail by Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, pp. 956-957, 1997.
In the related art SLS method, the amount of laser energy, the irradiation range of the laser beam, and the translation distance are controlled to permit lateral growth of a silicon crystal particle with a predetermined length, thereby crystallizing the amorphous silicon into a single crystal of 1 μm or more.
The irradiation device used for SLS concentrates the laser beam into a small and narrow region, and the amorphous silicon layer deposited on the substrate thus cannot be completely converted into polycrystalline silicon with a single irradiation. Therefore, in order to change the irradiation position on the substrate, the substrate having the amorphous silicon layer deposited thereon is mounted on a stage. Then, after irradiation on a predetermined area, the substrate is moved so as to allow irradiation to be performed on another area, thereby carrying out the irradiation process over the entire surface of the substrate.
FIG. 2 illustrates a schematic view of a related art sequential lateral solidification (SLS) device. FIG. 2 shows the related art sequential lateral solidification (SLS) device that includes a laser beam generator 1 generating laser beams, a focusing lens 2 focusing the laser beams discharged from the laser beam generator 1, and a mask 3 to dividedly irradiate the laser beam on a substrate 10. A reduction lens 4 formed below the mask 3 reduces the laser beam passing through the mask 3 to a constant width.
The laser beam generator 1 generally produces light with a wavelength of about 308 nanometers (nm) using XeCl or a wavelength of 248 nanometers (nm) using KrF in an excimer laser. The laser beam generator 1 discharges an unmodified laser beam. The discharged laser beam passes through an attenuator (not shown), in which the energy level is controlled. The laser beam then passes through the focusing lens 2.
The substrate 10 has an amorphous silicon layer deposited thereon, and the substrate 10 is fixed on an X-Y stage 5 that faces the mask 3.
In order to crystallize the entire surface of the substrate 10, the X-Y stage 5 is minutely displaced, thereby gradually expanding the crystallized region.
The mask 3 includes an open part ‘A’ that allows the laser beam to pass through, and a closed part ‘B’ blocks the laser beam (see FIG. 3). The width of the open part ‘A’ determines the lateral growth length of the grains formed after the first exposure.
FIG. 3 shows a plane view of a mask used in a laser irradiation process. FIG. 4 shows a crystallized region formed by a laser beam irradiation by using a mask of FIG. 3. Referring to FIG. 3, the mask used in the laser irradiation process is formed with the open part ‘A’ having patterns opened at a first interval (a), and the closed part ‘B’ has patterns closed at a second interval (b). The open and closed parts alternate sequentially.
The laser irradiation process using the mask will be described as follows.
First, the mask 3 is placed over the substrate having an amorphous silicon layer deposited thereon, and then the first laser beam is irradiated. At this time, the irradiated laser beam passes through the multiple open parts ‘A’ of the mask 3, whereby predetermined portions 22 of the amorphous silicon layer corresponding to the open parts ‘A’ are melted and liquefied, as shown in FIG. 4. In this case, the intensity of laser energy has a value selected from the complete melting region, so that the silicon layer irradiated with the laser completely melts.
At this time, by a single laser beam irradiation, the multiple open parts ‘A’ of the mask 3 correspond to one unit area 20 of the substrate, to which the laser beam irradiated, where the unit area 20 has a length ‘L’ and a width ‘S’.
After the laser beam irradiation, silicon grains 24a and 24b grow laterally from interfaces 21a and 21b between the amorphous silicon region and the completely liquefied silicon region, and the grains grow towards the irradiation region. The lateral growth of the silicon grains 24a and 24b proceeds in a perpendicular direction to the interfaces 21a and 21b. 
In the predetermined portion 22 irradiated with laser corresponding to the open part ‘A’ of the mask, if the width of the predetermined portion 22 is narrower than two times the growth length of the silicon grains 24a, then the grains growing inward in a perpendicular direction from both sides of the interface of the silicon region come into contact with one another at a grain boundary 25, thereby causing the crystal growth to stop.
Subsequently, in order to further grow the silicon grains, the stage bearing the substrate is moved to perform another irradiation process on an area adjacent to the first irradiated area. Another crystal thus forms with the new crystal being connected to the crystal formed after the first exposure. Similarly, crystals are laterally formed on each side of the completely solidified regions. Generally, the crystal length produced by the laser irradiation process and connected to the adjacent irradiated part is determined by the width of open part ‘A’ and closed part ‘B’ of the mask.
FIG. 5 illustrates an overlapped portion after completing the crystallization process over the entire surface of the substrate by using the mask of FIG. 3.
FIG. 5 shows that on progressing the crystallization by the unit areas (C1, C2, . . . , Cm, Cm+1, . . . ) of the substrate irradiated with shots of the laser beam, there are overlapped portions (01, 02) of laser beam irradiation on the substrate. That is, when irradiating the laser beam at the adjacent unit areas, the predetermined portions between the adjacent unit areas may be irradiated with the laser beam two (or more) times according to the open part ‘A’ of the mask that partially overlaps the adjacent unit area. For example, the laser beam irradiates in a condition where the substrate is moved along the X-axis direction at a distance corresponding to the length ‘L’ of the open part of the mask 3, and thus an overlapped portion 01 generates. Also, the laser beam irradiates under the condition where the substrate is moved along the Y-axis direction at the distance corresponding to (a+b)/2 of the mask 3, and thus an overlapped portion 02 generates. Among the overlapped portions 01 and 02, there are the overlapped portions 51 and 52 that are twice irradiated with the laser beam along any one direction of the X-axis or the Y-axis. Also, the overlapped portion 53 is irradiated with the laser beam four times along the X-axis and the Y-axis directions.
If circuit or display components are positioned in the overlapped portions 51, 52, and 53, the electron mobility may decrease due to non-uniformity of grains generated during the silicon crystallization process. Also, the picture quality degrades when the overlapped portions 51, 52, and 53 are in correspondence with the pixel region of display area.
Accordingly, the related art silicon crystallization method has the following disadvantages.
The related art silicon crystallization is performed over the entire surface of the substrate, and the silicon crystallization process proceeds without an additional alignment key. As a result, it is difficult to control the position of laser beam overlapped portions. Accordingly, if the laser beam overlapped portions correspond to the pixel regions or a channel region, it may cause deleterious low picture quality and low operation speed.
Generally, an LCD device is defined as the display area and a non-display area. Also, the predetermined portion of the LCD device requiring silicon crystallization corresponds to the portion having components necessary for rapid operating speed. That is, the portion requiring the silicon crystallization bears the components such as the display area for the driving circuit part (gate driver and data driver), and the non-display area is for the thin film transistor. Accordingly, it is possible to selectively perform the silicon crystallization process to a predetermined portion without applying the silicon crystallization process over the entire surface of the substrate. With the selective silicon crystallization, it is possible to decrease the time and number of laser irradiations. However, for selective silicon crystallization, it becomes necessary to provide an alignment key for sensing the portion of the substrate irradiated with the laser beam. For this, there arises a requirement to perform photolithography to form the additional alignment key, thereby placing a burden on the silicon crystallization process.